In February, 1984, the DTM converted from manual to automated platelet collection techniques, acquiring for this purpose four automated cell separation devices. During the next 3 years, 11 donors undergoing apheresis procedures on the Fenwal CS-3000 device experienced acute hypersensitivity reactions. Nine reactions occurred during plateletpheresis and 4 reactions occurred during granulocytapheresis procedures. The purpose of this study was to identify the allergen responsible for these reactions. Using a combination of skin tests, radioallergosorbent tests, and basophil histamine release assays, specific IgE- mediated sensitization to ethylene oxide, a gas used to sterilize the plastic disposable apheresis kits, was found in all plateletpheresis donors experiencing reactions, but in none of 100 non-reacting controls. Preliminary data also suggests that ethylene oxide is the agent responsible for hypersensitivity reactions during granulocytapheresis. It is estimated that as many as 1.0% of all repeat apheresis donors may become sensitized to ethylene oxide and experience allergic symptoms during donation. Although none of these reactions have been life- threatening, the DTM is continuing to test apheresis donors from sensitization to ethylene oxide, and to defer sensitized individuals from subsequent CS-3000 donations.